Background of the Invention
[0001] The present invention relates generally to CRT electron beam deflection systems,
and more particularly to the compensation of delays inherent in electromagnetic deflection
systems.
[0002] Television and other CRT-type image display systems typically use a large magnetic
yoke to scan the CRT's electron beam(s) over the surface of its phosphor display screen
in a raster pattern. Most multicolor cathode-ray tubes have three closely-spaced beams,
which must be substantially coincident, or converged, at the screen and remain so
as they are deflected over its surface. Thus, in addition to deflection by the common
magnetic yoke to produce a scanned raster, each beam is also deflected individually
as needed to maintain convergence over the entire screen. This small angle, variable
beam deflection, referred to as dynamic convergence, is accomplished by applying correction
signals to convergence coils on the neck of the CRT.
[0003] There is an inherent delay (not a pure delay, but instead a complex error function
the major part of which is a time shift) between the input current waveform driving
a deflection coil and the flux it produces. In a television-type display, the corrections
required for beam convergence and for pincushion and tilt distortion of the vertical
raster can be significantly affected by such delays. Most such displays operate at
fixed, relatively low scan rates -approx. 15 KHz for U.S. commercial television --
at which delays can be tolerated, or compensated using standard controls. However,
electromagnetically- deflected CRT displays operating at higher horizontal scan rates
encounter a much more severe problem, particularly those designed to operate over
a wide range of frequencies. For. example, a delay of six percent of the horizontal
width at 60 ps active time (approx. 15 KHz) would increase to 25% of the horizontal
width at 14ps active time (approx. 50 KHz). Video type display devices operating over
a wide range of horizontal scan rates thus require some means of compensating for
the effects of deflection system delays on beam position-related correction signals,
such as those for beam convergence or top and bottom raster tilt.
Summary of the Invention
[0004] The present invention solves the above-identified problem by advancing the correction
signals an amount sufficient to cancel out the deflection system delay(s). The required
advance may be provided, for example, through the use of a phase lock loop, but a
preferable approach is to add a d.c. level shift to a beam position-related signal
from which the correction signal is derived.
[0005] According to a preferred embodiment of the invention, which is described in greater
detail below, a correction waveform generator for an electromagnetically-deflected
video display is compensated for deflection system delays by adding an appropriate
d.c. level shift to a ramp signal from which the correction waveforms are derived.
This d.c. offset causes the correction signals to be time advanced an amount sufficient
to compensate for delays in the deflection system and its drive circuitry. This delay
compensation method -- i.e., adding a d.c. level shift to a beam position-related
ramp signal from which the correction signal is derived -- may be used for both the
beam convergence and the vertical raster top and bottom tilt correction signals.
[0006] Further applications of the present invention will become apparent as the following
detailed description is read in conjunction with the accompanying drawing.
Brief Description of the Drawing
[0007]
Fig. 1 is a simplified circuit diagram used in explaining the principle of the present
invention;
Fig. 2 is a time plot of input current Ic(t) in the Fig. 1 circuit;
Fig. 3 shows a simplified coil driver circuit;
Fig. 4 is a block diagram of a beam deflection correction system incorporating the
present invention;
Fig. 5 depicts certain waveforms illustrating the delay compensation provided by the
Fig. 4 system; and
Fig. 6 is a schematic diagram of certain circuitry for adapting the Fig. 4 system
to correction of beam convergeance.
Detailed Description
[0008] Referring now to the drawings, FIG. 1 illustrates a simplified model of an electromagnetic
deflection circuit that includes a CRT deflection coil L, a loss element R representing
the effective parallel resistance associated with the coil, and a suitable current
source S. It can be shown that for an input current I (t), where

the current I
L(t) in inductor L is

The gain coefficients K'
2, K'
3, K'
4, etc. are small with respect to the coefficients of the corresponding powers of t
in I
c (t-L/R) when L/R « t
A/2, t
A being the active time of the CRT's horizontal sweep.
[0009] FIG. 2 illustrates graphically the current I (t) generated by source S. By beginning
generation of the desired correction waveform (here a parabola) by suitable time -t
l prior to the start of the electron beam's active sweep period, which begins at time
t=o at the left side of the display screen (X = -1), the transient resulting from
the expoential term K
ee
-R/Lt will be substantially decayed. The field-producing current through the coil thus
may be approximated by I
L(t) ≈ I
C (t-L/R). Accordingly, the delay produced by loss element R can be compensated by
a time advance term +L/R.
[0010] The coil driver circuit shown (in simplified form) in FIG. 3 can be adjusted to cancel
the delay produced by the deflection coil's loss element R. The circuit includes an
operational amplifier OA whose noninverting input is referenced to ground by a resistor
R
O. An input resistor R
1 connected to the amplifier's inverting (-) input is bridged by a capacitor C, and
the output of the amplifier is coupled to its inverting input by. a feedback resistor
R
2. If the value of capacitor C is equal to L/R
1.R, the delay caused by loss element R will be cancelled out. The FIG. 3 circuit,
which acts as a differentiator, has certain drawbacks, however. It is less stable
than desired, and does not compensate for other delays in the deflection system.
[0011] A block diagram of a CRT beam deflection system illustrating the preferred practice
of the invention is shown in FIG. 4. Such a system may be used, for example, to achieve
dynamic convergence correction of the beams in a delta-gun color CRT. The FIG. 4 deflection
system uses beam position information to generate suitable correction waveforms, which
are applied to a deflection coil mounted on the CRT. The correction function may be
expressed as C(X,Y), where C(0,0) is the center of the display screen. The terms X
and Y represent positions on the horizontal and vertical axes, respectively, of the
CRT screen, each term varying in value between -1 and +1.
[0012] The FIG. 4 system includes a pair of adders 10 and 12 for summing beam-shifting signals
Xs and Ys with horizontal and vertical beam position signals X and Y, respectively.
The summed beam position and time shift signals are applied to the appropriate inputs
of a correction waveform generator 14, which produces a desired correction signal
C(X,Y) at its output. Generator 14 may, for example, be a convergence waveform generator
of known design, such as the one shown in U.S. Patent 3,942,067 to Cawood.
[0013] The output signal C(X,Y) from correction generator 14 is supplied to a coil driver
circuit 16, which provides an output current signal I
c(t) to drive a beam deflector 18. Deflector 18 includes a deflection coil L and its
associated effective parallel resistance R. Driver circuit 16 is conventional and
may, for example, take the form of a class B linear transconductance amplifier. The
current through coil L produces a magnetic field B that is coupled into CRT 22 to
provide a deflecting field B
d in the path of a beam within the tube. Color CRTs typically include internal pole
pieces near each electron gun for applying convergence correction fields to the beams.
The delay t
D produced by the internal coupling structure is represented in FIG. 4 by a delay block
20.
[0014] The beam position-related signals X and Y are related to time by the expressions

where t
HA is the horizontal active time (the time required for the beam to travel across the
screen from X = -1 to X = +1) and t
VA is the vertical active time. The beam shifting signals Xs and Ys are related to time
by the expressions

where ts is the time advance required to compensate for system delays.
[0015] As will be understood from FIG. 4,

and

When a beam shift Xs is included,

and

When ts is equal to (t + L/R),

Therefore,

is the horizontal shift required to cancel the time delays inherent in the FIG. 4
deflection system. A similar derivation may be made for Ys; however, in most systems
the vertical position shift may be omitted since Ys = L/R
tVA <<<
1.
[0016] As will by now be evident, the correction signal C(X,Y) may be advanced in time to
cancel out deflection system delays by suitably shifting the beam position-related
signal(s) from which the correction waveform is derived. This is accomplished, according
to the invention, by applying a d.c. offset to horizontal (or vertical) ramp signals
used to generate the correction signals. Referring to FIG. 5, it will be seen that
the addition of a d.c. offset Xs to a horizontal ramp signal X effectively advances
the ramp (i.e., shifts it toward the left side of the screen) an amount ΔX corresponding
to a time shift ot. The effect on a resultant parabolic correction signal C(X,Y) from
generator 14 (FIG. 4) is shown in the lower portion of the figure. Thus, by applying
an appropriate offset to the input ramp, delays inherent in the deflection coil and
its associated magnetic structure, as well as those associated with the coil drive
circuitry, may be readily compensated. The amount of offset, or time shift, required
will be different at different scan rates, and may be varied either mannually or automatically.
Moreover, an offset ramp signal used to correct for delays in one deflection system
may be used to correct for delays in a different system associated with the same CRT.
[0017] FIG. 6 illustrates suitable circuitry for the adder 10 and correction generator 14
of FIG. 4 as used in a convergence correction system for a color CRT display. Adder
10 includes an operational amplifier 26 receiving at its inverting input a horizontal
beam position signal (X) via input terminal 24 and input resistor 28. The non-inverting
input of amplifier 26 is referenced to ground potential by resistor 27. A feedback
resistor 30 paralleled by a capacitor 36 is connected between the output and inverting
input of the amplifier. A d.c. position shift signal (X ) generated by a potentiometer
32 is applied to the inverting input of amplifier 26 via series resistors 33, 34,
whose common junction may be connected to ground via a switch 35.
[0018] It will be understood that the output signal from operational amplifier 26 is of
the form (X + X
S). For convenience, however, the combined signal is indicated simply by X in FIG.
6. The position control signal (X ) is controllable by potentiometer 32 to any value
required to cancel out delays in the system, or may be disabled by closing switch
35.
[0019] Correction generator 14 includes three multipliers 38, 40, and 42, five differential
operational amplifiers 44, 46, 48, 50 and 52, two inverters 54, 56, a variable gain
amplifier (or attenuator) 58, two potentiometers 60 and 62, and associated passive
elements, which are configured to provide a parabola signal KIX2, and two higher degree
correction signals K
2(X
2 - X
4) and K
3(X - X
3). These are, in turn, supplied to the input of summing amplifier 52 to provide the
required convergence correction signal from output terminal 64.
[0020] The functions (X - X
4) and (
X - X
3) are used to provide more precise correction without causing interaction with the
parabola signal K
1X
2 at the right and left sides of the screen.
[0021] It will be apparent to those skilled in the art that many changes and modifications
may be made in the specific circuits and examples given herein. Such variations are
not to be regarded as a departure from the scope of the invention, which is limited
only as required by the terms of the appended claims and the supporting disclosure.
1. An electron beam deflection circuit for a cathode ray tube, the circuit including
a deflection coil and a signal generator coupled to the coil, the generator providing
an' output signal derived from a beam position-related input signal, characterized by
the addition to the input signal of a further signal to effect a time advance of the
output signal sufficient to substantially cancel time delays inherent in the deflection
circuit.
2. The circuit of claim 1, wherein said beam position-related signal is a ramp signal,
and said further signal is a d.c. potential.
3. An electromagnetic deflection circuit for deflecting an electron beam being generated
in the neck of a cathode ray tube, said deflection circuit having inherent time delays
inclusive therein due to a plurality of loss elements in said deflection circuit,
comprising:
means for developing an input signal, said input signal energizing said electromagnetic
deflection circuit, the means for developing an input signal including,
means for generating a beam position input signal, means for generating a shift signal
representative of a DC level increment; and
adder means responsive to said beam position input signal and to said shift signal
for adding said shift signal to said beam position input signal thereby introducing
said DC level increment into said beam position input signal and producing said input
signal in response thereto; and
means responsive to said input signal for developing a magnetic field, said magnetic
field deflecting said electron beam being generated in the neck of said cathode ray
tube;
whereby the introduction of said DC level increment into said beam position input
signal compensates for the inherent time delays produced by the plurality of loss
elements in said deflection circuit.
4. An electromagnetic deflection circuit in accordance with claim 3 wherein three
electron beams are generated in the neck of said CRT tube; and wherein the means for
developing a magnetic field comprises:
correction signal generating means responsive to said input signal for generating
a correction signal representative of the amount of correction needed to converge
said three electron beams, said correction signal being advanced along the time axis
in response to said shift signal representative of said DC level increment.
5. An electromagnetic deflection circuit in accordance with claim 4 wherein the means
for developing further comprises;
means responsive to said correction signal for amplifying said signal; and
deflection means responsive to the amplified correction signal for generating a magnetic
field, said magnetic field deflecting at least one of the electron beams being generated
in the neck of said cathode ray tube.
6. An electromagnetic deflection circuit in accordance with claim 5 wherein the means
for amplifying comprises an operational amplifier, one input terminal being connected
to a ground potential, another input terminal. being responsive to said correction
signal, said another input terminal being connected to a resistor-capacitor parallel
combination, the output terminal of said operational amplifier and said another input
terminal being connected together via a feedback resistor.